GemRB is a portable open-source implementation of the Infinity Engine
Cooperative multiplayer graphical RPG and adventure game
KildClient is a powerful MUD client with a built-in Perl interpreter
General Purpose Engine for Graphic Adventures.
NetHack made ready for internationalization
Cross platform 2D game creation framework with Python bindings
realistic but simple flight simulator: exploration and casual landing
War of Respect is an online turn-based MMORPG in 2D-anime style.